Redox flow batteries (RFBs) are promising electrochemical devices for grid-scale energy storage due to their many favorable inherent attributes, such as decoupled power and energy ratings, as well as excellent performance and capacity retention. Recently, significant improvements in cell performance have made RFB systems more viable than ever. However, to meet the U.S. Department of Energy long range capital-cost target of $150 kWh^-1, additional cost reductions are needed. One promising route is new active species for RFBs designed for lower cost or improved performance. These next-generation RFB chemistries are likely to be engineered molecules or complexes, such as redox-active organic or organometallic compounds, which offer a multitude of possibilities. A key consideration in the selection of any new RFB chemistry is the supporting electrolyte, which has a significant influence on the membrane resistance, electrolyte conductivity, and electrolyte viscosity. All of these physical parameters have a major impact on RFB performance and cost. Therefore, it is useful to quantify changes in economic viability for various RFB chemistry options with different aqueous supporting electrolytes paired with different types of membranes, which is the focus of this work. A techno-economic model is used to estimate RFB-system costs for the different membrane and supporting electrolyte options considered herein. Variations in cell performance due to the working ion selection and electrolyte viscosity can yield battery cost differences in the $100’s kWh^-1, and this analysis allows for quantification of cost performance changes by selecting certain electrolyte characteristics. Beyond the conventional RFB design incorporating small active species and an ion-exchange membrane (IEM), this work also considers size-selective separators (SSS) as a cost-effective alternative to IEMs. The SSS concept utilizes nano-porous materials with no functionalization for ion selectivity, and active species that are too large to pass through the pores. Across the entire design space, SSS separators with low viscosity electrolytes offer the lowest RFB costs. Acknowledgements

This work was supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the United States Department of Energy. J.D.M. acknowledges additional financial support from the National Science Foundation Graduate Research Fellowship Program.